The lubricant oil used for machining, also herein termed cutting oil, is admixed with sulfur-based extreme-pressure additives or chlorine-based extreme-pressure additives. However, if the cutting oil admixed with sulfur-based extreme pressure additives or chlorine-based extreme pressure additives is used for machining materials difficult to cut, such as Inconel, the tool has a short service life, such that the tool exchange time is sometimes longer than the cutting time. On the other hand, if the workpiece difficult to cut, such as Inconel, is bored by a cemented carbide drill, only a few holes at most can be bored with a sole drill.
In machining a soft material, poor in thermal resistance and mold releasing properties, cutting chips remain loaded to the cutting edge of the tool thus lowering machining precision and yield. Since it is necessary in this case to lower the machining speed to prevent the temperature from increasing or to exchange the tools frequently. If the tool is exchanged frequently, the productivity is lowered. Thus, in boring an aluminum material, as an example, the number of revolutions of the drill needs to be suppressed to a lower value of the order of several hundred revolutions per minute.
In cutting oils, for which thermal resistance is also a requirement, those admixed with graphite or molybdenum disulfide, as solid lubricants, are sometimes used. However, the thermal resistance of graphite or molybdenum disulfide is 500 to 600.degree. C., such that, if this temperature is exceeded, loading occurs frequently to lower the machining yield. Although the lubricating performance may be improved to a certain extent with the conventional cutting oil admixed with molybdenum disulfide or graphite, it is not possible to realize significant improvement in the operating efficiency or machining precision or in durability, such that definite limits are set in improving productivity or in reducing the machining cost.
Boron nitride (BN), a compound composed of boron and nitrogen, has many different forms having substantially the same crystal structures as carbon. Carbon exists as amorphous carbon, graphite of the hexagonal structure having a laminated structure of hexagonally-shaped meshed layers, and diamond of the cubic system. Of these, graphite of the hexagonal system, having a laminated structure of hexagonal meshed layers, and exhibiting significant cleavage characteristics at the inter-layer portions, exhibits the solid lubricating performance. Boron nitride also is known to have different forms, such as amorphous boron nitride (referred to herein as a-BN), boron nitride of the hexagonal system having a laminated structure of hexagonal-shaped meshed layers at a repetitive period of two layers (referred to herein as h-BN), rhombohedral structure having a laminated structure of hexagonal-shaped meshed layers at a repetitive period of three layers (referred to herein as r-BN), a turbostratic boron nitride having randomly layered hexagonal-shaped meshed layers (referred to herein as t-BN) and a high pressure phase cubic born nitride of the cubic structure (referred to herein as c-BN).
The h-BN crystals are known to exhibit cleavage characteristics, comparable to those of graphite crystals of the hexagonal system, and hence optimum solid lubricating properties. The lubricating properties of the h-BN crystals are presumably ascribable to the Van-Del-Waals bond with a weak bond between the two-dimensional hexagonal meshed layers comparable to that of graphite. That is, the crystals exhibit significant cleavage characteristics along this plane such that crystal grains cleft in flakes between the layers are liable to slip relative to one another.
A sintered mass of h-BN powders of high purity is colorless to white in color, superior in electrical insulating properties, higher in resistance against oxidation than graphite, less liable to be reacted with and melted into ferrous materials like graphite and are less apt to suffer burning to the ferrous materials because of a reduced ractivity with the ferrous material. In this regard, h-BN is a suitable material as a solid lubricant material.
As a typical case of exploiting the lubricating properties of h-BN, JP Patent Kokai JP-A-63-135496 discloses a lubricant oil excellent in thermal resistance and in friction reducing effect, obtained by dispersing in a fluid oil and fat h-BN powders and polyether ketone powders both of which are not larger than 20 .mu.m in the mean particle size. On the other hand, JP Patent Kokai JP-A-01-318087 discloses lubricant oil which is excellent in lubricating properties and in sliding performance and which is obtained on mixing and dispersing in e.g., silicone oil h-BN powders with a particle size of 2 to 10 .mu.m and used for drawing steel wires etc.
On the other hand, the a-BN powders are hygroscopic and unstable and hence are not suited as boron nitride powders admixed to the lubricant oil, so that h-BN powders, exhibiting no hygroscopicity, are predominantly used. However, the h-BN powders are costly and hence are used only for special application in which cost increase is tolerable. As far as the present inventors have searched, there is no instance of use of cutting oils or grinding oils admixed with the boron nitride powders. It is noted that r-BN or t-BN has barely reached the stage of tentative manufacture in a laboratory such that discussions of the practical usage are thought to be of no avail.
In the Journal of the Shigen-Sozai Gakkai (Society of Resources and Materials), vol.1. 105, No.2, p.201, 1988, a-BN is explained as being t-BN. However, the powder X-ray diffraction diagram by CuK.alpha. rays of boron nitride powders, termed t-BN therein, demonstrates only two broad lines of diffraction at the positions of [100] and [101] neighboring to a diffraction peak [002] of h-BN, while not demonstrating or scarcely demonstrating the diffraction peak at the position of the diffraction peak [004]. In the present specification, the position of the diffraction peak in the powder X-ray diffraction diagram of boron nitride powders is represented as an index of the diffraction peaks characteristic to h-BN for convenience of explanation. This powder X-ray diffraction diagram is similar to that for a-BN shown in FIG. 1. Therefore, it is not proper to assume that a-BN be the same as t-BN.